Conventionally, there is known a voltage follower circuit having a structure in which an output of a source follower output transistor is fed back to the gate electrode of the source follower output transistor via a differential amplifier. An example of such voltage follower circuit is shown in FIG. 5. From FIG. 5, it can be seen that an output n-channel type MOS transistor 26, a resister 9 and a resistor 10 are serially connected between a high potential power supply voltage line 51 and the ground line 52, and thereby a source follower output, stage is constituted. Here, the voltage of the high potential power supply voltage line is designated as Vbatt. The source electrode and the backgate electrode of the transistor 26 are mutually coupled, and a source potential voltage is applied to the backgate electrode. The circuit further comprises a differential amplifier 1 which receives a reference voltage Vref supplied from an external source at a non-inverting input thereof. A potential voltage at the connection point between the resistor 9 and the resistor 10 of the source follower output stage is returned to an inverting input of the differential amplifier 1, and the output point of the differential amplifier 1, that is, node A, is coupled to the gate electrode of the source follower output transistor 26. That is, this circuit constitutes a voltage follower structure by using a negative feedback of a divided voltage of an output voltage Vout as a gate input of the output transistor, via the differential amplifier.
The voltage follower circuit shown in FIG. 5 has a disadvantage in that, when the power supply voltage Vbatt is a high voltage, the area occupied by the circuit becomes large. The reasons for this disadvantage will be described below.
When a LSI including the voltage follower circuit shown in FIG. 5 is used, for example, as an IC for mobile use, the power supply voltage Vbatt is higher than a voltage used for a usual LSI, for example, 5V or 3.3V, and becomes a voltage of approximately between 7 through 40V. What voltage is used as the power supply voltage Vbatt is determined depending on the type of a car, for example, depending on whether a car using the LSI is a passenger car or a truck, and so on. Therefore, the power supply voltage of the LSI for mobile use is selected to be the highest voltage, i.e., 40V in the above-mentioned voltage range between 7V and 40V. This is because, in order to cope with various types of cars by using one type of LSI, it is necessary to guarantee that the LSI can safely function even in the highest power supply voltage.
In FIG. 5, an output voltage range of the amplifier 1 is approximately from the power supply voltage Vbatt (=40V, in case of this example) to the ground potential level. Therefore, depending on the value of the output voltage Vout, for example, when the output terminal 53 is instantaneously short-circuited with the ground, when the output voltage Vout is not yet settled just after the power supply voltage is turned on, and so on, there is a possibility that a voltage which is maximally equal to the power supply voltage Vbatt is applied between the gate electrode and source and backgate electrodes of the source follower transistor 8. To this end, in the circuit of FIG. 5, all the MOS transistors used in the LSI must be high withstanding voltage transistors. Thus, a gate oxide film of each MOS transistor must be thick and also, at the same time, channel length must be long. Consequently, current drive ability of the MOS transistor, shown in the expression below, is deteriorated. In order to guarantee a large current drive ability of the transistor, channel width of the transistor must be made large, and when the LSI is required to have high withstanding voltage and especially large output current, the area occupied by the voltage follower circuit becomes very large. EQU Id=(1/2)*(W/L).mu..sub.0 (.di-elect cons..sub.ox /t.sub.ox)*(Vgs-Vt).sup.2
(Where, Id: drain current, W: channel width, L: channel length, .mu..sub.0 : mobility of carrier, .di-elect cons..sub.ox : dielectric constant of oxide film, t.sub.ox : thickness of gate oxide film, Vgs: gate voltage, Vt: threshold voltage of MOS transistor).